Can I save a partially rendered image and continue rendering it later?
As far as I know, there is no way of pausing or stopping a render to resume later.
You can pause the cycles rendering in the realtime viewport, but not the final render.

The most you can do is 'pause' the whole application at OS level. But it doesn't solve the 're-render later' problem, and if it crashes anyway, same situation.
You might find this post useful too.

Firstly, I'd suggest using a render farm. Pixel Plow look to be the cheapest at the moment. You can use the cost calculator but it's looking like it would cost around $2 to render what you've got.

Another solution is to render it out in smaller parts using render region.
Go to output properties and tick 'render region'



Then switch to camera view, and you can drag the corners/edges to set the region to render.



What you could do is to render out regions and stitch together later.
So you could render maybe full height strips, say 8 or so, overlapping a bit to be safe. Then at least if you crash you'll only lose 2.5 hours.

But, for time it takes to do all that faffing, wait for it to render, stitch it all together in photoshop later etc. you might be better off spending a couple of dollars on render farm.